This disclosure relates generally to the field of nonelectric reusable heat sources.
Devices known as heat pads and heat packs have been long used for many different applications. As one example, such devices are commonly used for physical therapy or pain management, by applying heat to a localized portion of a person's body. As another example, such devices are commonly used to extend the period of time that an individual can spend comfortably outside during cold weather conditions.
There are several basic types of heat pads and heat packs, each employing a different technology. Electrical heating pads can provide heat at a desired temperature for an indefinite period of time, but require an electrical power source. Some heating pads and heating packs use a material having a high specific heat capacity to provide heat for a limited period of time immediately after being heated. A hot water bottle is an example of a device that uses a high specific heat material, namely water, to provide heat immediately after the water is heated and placed in the bottle.
Chemical heat packs are known that remain at room temperature until activated, causing an exothermic reaction that lasts for a period of time after activation. Some chemical heat packs can be used only once, and are non-reusable. An example of a single-use chemical heat pack employs a binary mixture where a chemical reaction is initiated by mixing the components of the binary mixture. Other chemical heat packs are reusable. One common type of reusable chemical heat pack utilizes a phase change material, such as a supercooled solution of sodium acetate (CH3COONa) in water, where the phase change material crystallizes exothermically when activated. The heat pack is prepared for subsequent use by melting the phase change material, such as by immersing the heat pack in hot water.